IIT JEE Physics Formula Revision 28. Heat transfer

1. ∆Q/∆t = KA(T1-T2)/x



2. ∆Q/∆t = -KAdT/dx

3. x/KA is called the thermal resistance R.
∆Q/∆t is termed as heat current i

i = T1-T2/R

4. In the experimental determination of thermal conductivity of a solid

K = [xms(θ3-θ4)]/[A(θ1-θ2)t]

5. Wien's displacement law
λ_mT = b = constant

6. Stefan-Boltzmann Law

The energy of thermal radiation emitted by per unit time by a black body of surface area A is given by

U = σAT⁴

Where
σ = Stefan Boltzmann constant = 5.67*10^-8 W/m²-K⁴


The energy of thermal radiation emitted by per unit time by a body (not black) of surface area A is given by

U = eσAT⁴

e is a constant for the given surface called emissivity of the surface. Its value is between 0 and 1. It is one for black body and 0 for completely reflecting surface.

7. Net loss of thermal energy of body with temperature T kept in a room at temperature T₀

u = eσA(T⁴ - T₀⁴)


8. Newton’s law of cooling

dT/dt = -bA(T-T₀)

b = a constant depends on the nature of the surface involved
a = surface area exposed of the body
T- T₀) = temperature difference between the body and surrounding